Swans are birds of the familyAnatidae within the genus Cygnus.[3] The swans' close relatives include the geese and ducks. Swans are grouped with the closely related geese in the subfamilyAnserinae where they form the tribeCygnini. Sometimes, they are considered a distinct subfamily, Cygninae. There are six or seven living (and one extinct) species of swan in the genus Cygnus; in addition, there is another species known as the coscoroba swan, although this species is no longer considered one of the true swans. Swans usually mate for life, although “divorce” sometimes occurs, particularly following nesting failure, and if a mate dies, the remaining swan will take up with another. The number of eggs in each clutch ranges from three to eight.[4]

The English word 'swan', akin to the GermanSchwan, Dutchzwaan and Swedishsvan, is derived from Indo-European root*swen (to sound, to sing).[5] Young swans are known as swanlings or as cygnets; the latter derives via Old Frenchcigne or cisne (diminutive suffix -et "little") from the Latin word cygnus, a variant form of cycnus "swan", itself from the Greekκύκνοςkýknos, a word of the same meaning.[6][7][8] An adult male is a cob, from Middle English cobbe (leader of a group); an adult female is a pen.[9]

A mute swan landing on water. Due to the size and weight of most swans, large areas of open land or water are required to successfully take off and land.

Swans are the largest extant members of the waterfowl family Anatidae, and are among the largest flying birds. The largest species, including the mute swan, trumpeter swan, and whooper swan, can reach a length of over 1.5 m (59 in) and weigh over 15 kg (33 lb). Their wingspans can be over 3.1 m (10 ft).[10] Compared to the closely related geese, they are much larger and have proportionally larger feet and necks.[11] Adults also have a patch of unfeathered skin between the eyes and bill. The sexes are alike in plumage, but males are generally bigger and heavier than females.[9]

The Northern Hemisphere species of swan have pure white plumage but the Southern Hemisphere species are mixed black and white. The Australian black swan (Cygnus atratus) is completely black except for the white flight feathers on its wings; the chicks of black swans are light grey. The South American black-necked swan has a white body with a black neck.[12]

Swans' legs are normally a dark blackish grey colour, except for the two South American species, which have pink legs. Bill colour varies: the four subarctic species have black bills with varying amounts of yellow, and all the others are patterned red and black. Although birds do not have teeth, swans have beaks with serrated edges that look like small jagged 'teeth' as part of their beaks used for catching and eating aquatic plants and algae, but also molluscs, small fish, frogs and worms.[13] The mute swan and black-necked swan have lumps at the base of their bills on the upper mandible.[14]

Several species are migratory, either wholly or partly so. The mute swan is a partial migrant, being resident over areas of Western Europe but wholly migratory in Eastern Europe and Asia. The whooper swan and tundra swan are wholly migratory, and the trumpeter swans are almost entirely migratory.[11] There is some evidence that the black-necked swan is migratory over part of its range, but detailed studies have not established whether these movements are long or short range migration.[15]

Swans feed in water and on land. They are almost entirely herbivorous, although they may eat small amounts of aquatic animals. In the water, food is obtained by up-ending or dabbling, and their diet is composed of the roots, tubers, stems and leaves of aquatic and submerged plants.[11]

Although swans only reach sexual maturity between 4 and 7 years of age, they can form socially monogamous pair bonds from as early as 20 months that last for many years,[16] and in some cases these can last for life.[17] The lifespan of the mute swan is often over 10 years, and sometimes over 20, whereas the black-necked swan survives for less than a decade in captivity.[18] These bonds are maintained year-round, even in gregarious and migratory species like the tundra swan, which congregate in large flocks in the wintering grounds.[19] Their nest is on the ground near water and about a metre across. Unlike many other ducks and geese, the male helps with the nest construction, and also take turns incubating the eggs,[20] and alongside the whistling ducks are the only anatids that will do this. Average egg size (for the mute swan) is 113×74 mm, weighing 340 g, in a clutch size of 4 to 7, and an incubation period of 34–45 days.[21]
Swans are highly protective of their nests. They will viciously attack anything that they perceive as a threat to their chicks, including humans. One man was suspected to have drowned in such an attack.[22][23]

Evidence suggests that the genus Cygnus evolved in Europe or western Eurasia during the Miocene, spreading all over the Northern Hemisphere until the Pliocene. When the southern species branched off is not known. The mute swan apparently is closest to the Southern Hemisphere Cygnus (del Hoyo et al., eds, Handbook of the Birds of the World); its habits of carrying the neck curved (not straight) and the wings fluffed (not flush) as well as its bill color and knob indicate that its closest living relative is the black swan. Given the biogeography and appearance of the subgenusOlor it seems likely that these are of a more recent origin, as evidence shows by their modern ranges (which were mostly uninhabitable during the last ice age) and great similarity between the taxa.
[24]

Mute swan, Cygnus olor, is a Eurasian species that occurs at lower latitudes than whooper swan and Bewick's swan across Europe into southern Russia, China and the Russian Maritimes. Recent fossil records, according to the British Ornithological Union, show Cygnus olor is among the oldest bird species still extant and it has been upgraded to "native" status in several European countries, since this bird has been found in fossil and bog specimens dating back thousands of years. Common temperate Eurasian species, often semi-domesticated descendants of domestic flocks, are naturalized in the United States and elsewhere.

Subgenus Chenopis

Black swan, Cygnus atratus of Australia, and introduced in New Zealand

Whooper swan, Cygnus cygnus breeds in Iceland and subarctic Europe and Asia, migrating to temperate Europe and Asia in winter

Trumpeter swan, Cygnus buccinator is the largest North American swan. Very similar to the whooper swan (and sometimes treated as a subspecies of it), it was hunted almost to extinction but has since recovered

Tundra swan, Cygnus columbianus is a small swan that breeds on the North American tundra, further north than the trumpeter swan. It winters in the USA.

Bewick's swan, Cygnus (columbianus) bewickii is the Eurasian form that migrates from Arctic Russia to western Europe and eastern Asia (China, Korea, Japan) in winter. It is often considered a subspecies of C. columbianus, creating the species tundra swan.

Whistling swan, Cygnus (columbianus) columbianus may be used to refer specifically to the North American form.

The fossil record of the genus Cygnus is quite impressive, although allocation to the subgenera is often tentative; as indicated above, at least the early forms probably belong to the C. olor – Southern Hemisphere lineage, whereas the Pleistocene taxa from North America would be placed in Olor. A number of prehistoric species have been described, mostly from the Northern Hemisphere. Among them was the giant Siculo-MalteseC. falconeri, which was taller (though not heavier) than the contemporary local dwarf elephants (Elephas falconeri).

The supposed fossil swans "Cygnus" bilinicus and "Cygnus" herrenthalsi were, respectively, a stork and some large bird of unknown affinity (due to the bad state of preservation of the referred material). Anser atavus is sometimes placed in Cygnus.[citation needed]

The coscoroba swan (Coscoroba coscoroba) from South America, the only species of its genus, is apparently not a true swan. Its phylogenetic position is not fully resolved; it is in some aspects more similar to geese and shelducks.[27]

Many of the cultural aspects refer to the mute swan of Europe. Perhaps the best known story about a swan is "The Ugly Duckling" fable. Swans are often a symbol of love or fidelity because of their long-lasting, apparently monogamous relationships. See the famous swan-related operas Lohengrin[28] and Parsifal.[29] Swan meat was regarded as a luxury food in England in the reign of Elizabeth I. A recipe for baked swan survives from that time: "To bake a Swan Scald it and take out the bones, and parboil it, then season it very well with Pepper, Salt and Ginger, then lard it, and put it in a deep Coffin of Rye Paste with store of Butter, close it and bake it very well, and when it is baked, fill up the Vent-hole with melted Butter, and so keep it; serve it in as you do the Beef-Pie."[30]

Swans feature strongly in mythology. In Greek mythology, the story of Leda and the Swan recounts that Helen of Troy was conceived in a union of Zeus disguised as a swan and Leda, Queen of Sparta.[31] Other references in classical literature include the belief that upon death the otherwise-silent mute swan would sing beautifully—hence the phrase swan song;[32] as well as Juvenal's sarcastic reference to a good woman being a "rare bird, as rare on earth as a black swan", from which we get the Latin phrase rara avis, rare bird.[33] The mute swan is also one of the sacred birds of Apollo, whose associations stem both from the nature of the bird as a symbol of light as well as the notion of a "swan song". The god is often depicted riding a chariot pulled by or composed of swans in his ascension from Delos.

The Irish legend of the Children of Lir is about a stepmother transforming her children into swans for 900 years.[34] In the legend The Wooing of Etain, the king of the Sidhe (subterranean-dwelling, supernatural beings) transforms himself and the most beautiful woman in Ireland, Etain, into swans to escape from the king of Ireland and Ireland's armies. The swan has recently been depicted on an Irish commemorative coin.

In Norse mythology, there are two swans that drink from the sacred Well of Urd in the realm of Asgard, home of the gods. According to the Prose Edda, the water of this well is so pure and holy that all things that touch it turn white, including this original pair of swans and all others descended from them. The poem Volundarkvida, or the Lay of Volund, part of the Poetic Edda, also features swan maidens.

A swan is one of the attributes of St Hugh of Lincoln based on the story of a swan who was devoted to him.[37]

In Latin American literature, the Nicaraguan poet Rubén Darío (1867–1916) consecrated the swan as a symbol of artistic inspiration by drawing attention to the constancy of swan imagery in Western culture, beginning with the rape of Leda and ending with Wagner's Lohengrin. Darío's most famous poem in this regard is Blasón – "Coat of Arms" (1896), and his use of the swan made it a symbol for the Modernismo poetic movement that dominated Spanish language poetry from the 1880s until the First World War. Such was the dominance of Modernismo in Spanish language poetry that the Mexican poet Enrique González Martínez attempted to announce the end of Modernismo with a sonnet provocatively entitled, Tuércele el cuello al cisne – "Wring the Swan's Neck" (1910).

Swans are revered in Hinduism, and are compared to saintly persons whose chief characteristic is to be in the world without getting attached to it, just as a swan's feather does not get wet although it is in water. The Sanskrit word for swan is hamsa and the "Raja Hamsam" or the Royal Swan is the vehicle of Goddess Saraswati which symbolises the "Sattva Guna" or purity par excellence. The swan if offered a mixture of milk and water, is said to be able to drink the milk alone. Therefore, Goddess Saraswati the goddess of knowledge is seen riding the swan because the swan thus symbolizes "Viveka" i.e. prudence and discrimination between the good and the bad or between the eternal and the transient. This is taken as a great quality, as shown by this Sanskrit verse:

Hamsah shwetah, bakah shwetah, kah bhedah hamsa bakayo?

Neeraksheera viveketu, Hamsah hamsah, bakah bakah!

The swan is white, the crane is white, so how to differentiate between them?

Observe the prudence, the swan is proven swan, the crane is proven crane!

It is mentioned several times in the Vedic literature, and persons who have attained great spiritual capabilities are sometimes called Paramahamsa ("Supreme Swan") on account of their spiritual grace and ability to travel between various spiritual worlds. In the Vedas, swans are said to reside in the summer on Lake Manasarovar and migrate to Indian lakes for the winter. They're believed to possess some powers such as the ability to eat pearls.

^Rees, Eileen. "6: Swans are one of the few species that can divert to homosexuality in times of loneliness. Mate fidelity in swans, an interspecific comparison". In Jeffrey M. Black, Mark Hulme. Partnerships in birds. Oxford: Oxford University Press. pp. 118–122. ISBN0-19-854860-5.